iprep: cleanup ctx on shutdown

~~Dr.M~~ Error #1: LEAK 480 direct bytes 0x0aae7fc0-0x0aae81a0 + 0 indirect bytes
~~Dr.M~~ # 0 replace_malloc                    [/work/drmemory_package/common/alloc_replace.c:2373]
~~Dr.M~~ # 1 SRepInit                          [.../Suricata/src/reputation.c:594]
~~Dr.M~~ # 2 DetectEngineCtxInit               [.../src/detect-engine.c:844]
~~Dr.M~~ # 3 main                              [.../Suricata/src/suricata.c:2230]